Monoclonal antibodies will be used to analyze three major processes in mammalian development - cell migration, differentiation, and determination. Migration: Evidence is presented that there are common features in the migration of two cell lineages from the yolk sac - the primordial germ cells and the hematopoietic stem cells, and that the specificity of these migrations resides in the interactions of these cells with the surfaces over which they migrate. Monoclonal antibodies against primordial germ cells will be produced in order to see if there are specific antigens shared by the two cell lines. These antibodies would be used for various biological tests to determine whether or not the antigens they recognize control the specificity of cell migrations. Differentiation: Evidence is presented that the classic transfilter induction of metanephric tubules by ureteric bud or spinal cord is due to cell-cell interactions. Hybridomas would be made from the spleen cells of those mice (injected with syngeneic spinal cord) whose sera inhibited transfilter tubulogenesis. The monoclonal antibodies would likewise be tested, and those positive sera would be absorbed with tissues known to be inducers or non-inducers of metanephric tubules. In this way, it could be determined whether other organ systems (such as the salivary gland) use the same antigenic stimuli. Determination: We seek to test a model of determination postulating that determination involves the differentiation of the nuclear membrane such that this membrane only allows certain nuclear RNAs to reach the cytoplasm. Monoclonal antibodies would be utilized to detect specific components peculiar to mouse liver nuclei. Once obtained, the antibodies would be used (a) to observe when these specific antigens arise during hepatic ontogeny, (b) to determine if these antigens are specific for the liver throughout development or if they are shared (say, by the yolk sac endoderm), and (c) to affinity-purify these antigens to the point where biochemical studies could be performed to analyze their functions and characteristics.